An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes, denoted as anode and cathode, surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. Oftentimes, the electrodes also function as the gas diffusion layer (or GDL) of the fuel cell. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell includes a membrane electrode assembly sandwiched between two flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which join with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen is removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
It has been disclosed that a graphite sheet that has been provided with through-channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expandable graphite, can be used to form gas diffusion layers for PEM fuel cells. As taught by Mercuri, Weber and Warddrip in U.S. Pat. No. 6,413,671, the disclosure of which is incorporated herein by reference, the through-channels can be formed in the flexible graphite sheet at a plurality of locations by a compressive mechanical impact, such as by use of rollers having truncated protrusions extending therefrom. The through-channel pattern can be devised in order to control, optimize or maximize fluid flow through the through-channels, as desired. For instance, the pattern formed in the flexible graphite sheet can comprise selective placement of the through-channels, or it can comprise variations in through-channel density or shape in order to, for instance, reduce or minimize flooding, control gas flow, restrict water flow, equalize fluid pressure along the surface of the electrode when in use, or for other purposes. See, for instance, Mercuri and Krassowski in International Publication No. WO 02/41421 A1.
Compressive force may also be used to form the continuous reactant flow channel in the material used to form a flow field plate (hereinafter “FFP”). Typically an embossing tool is used to compress the graphite sheet and emboss the channels along the surface of the sheet. Unlike, the GDL, the channel(s) in the FFP do not extend through the FFP from one opposed surface to a second surface. Typically, the channel(s) is on one surface of the FFP, although a cooling channel can be formed on the other surface, for the flow of a cooling fluid therealong.
In addition, and as taught by Mercuri et al. in U.S. Pat. No. 6,528,199, the disclosure of which is incorporated herein by reference, a combination GDL/FFP can be provided, wherein a reactant flow channel is formed in a graphite sheet that has been provided with channels. Therefore, both the fluid flow function of an FFP and the fluid diffusion function of a GDL can be combined in a single component.
Depending on the desired end use of the flexible graphite sheet, whether it be flow field plate, gas diffusion layer, catalyst support, or a non-fuel cell application such as heat sinks, heat spreaders or thermal interfaces for electronic thermal management applications, it may be necessary to emboss features on one or more surfaces of the sheet, such as flow field channels. Different methods have been proposed for providing embossed features with improved feature definition (see, for instance, U.S. Pat. Nos. 6,604,457 and 6,663,807, both to Klug; and International Publication No. WO 02/084760 A2, also to Klug). However, further optimization of the flexible graphite sheet material itself is still believed within reach.
What is desired, therefore, is a flexible graphite sheet material (and method for producing the material) formed so as to further facilitate the formation of embossed features on one or both surfaces thereof. The desired material should have characteristics which permit the embossing of features thereon while reducing the values of the process parameters of the embossing process. The desired material should also facilitate the functioning of the sheet material in its end-use application, such as a fuel cell component.
Graphites are made up of layered planes of hexagonal arrays or networks of carbon atoms. These layered planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion.
Graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles. These flexible graphite sheets can be described as sheets of compressed particles of exfoliated graphite.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.